Production of cyanuric acid



United States Patent 3,117,968 PRQDUCTESN SF QYANURIC ACll) Karl Merlrei and Albert Palm, Ludwigshafen (Rhine), Germany, assignors to Badische Anilin- 8r Sloda-Fabrik Alrfiengcsellschaft, Ludwigshafen (Rhine), Germany No Drawing. Filed Jan. 2, E62, Ser. No. 163,859 Claims priority, application Germany Get. 28, 1958 (liaims. (Cl. 260248) The present invention relates to an improved process for the separation or" cyanuric acid from the reaction mixture used in the conventional-type method of producing cyanuric acid by heating urea in a liquid.

In the technical-scale production of cyanuric acid by conventional methods the cyanuric acid formed separates from the liquid as a solid. Part of this solid settles down in crystalline form on the inner side of the reactor Wall whence it can be removed only with difliculty in a mechanical way.

The object of this invention is a method of producing cyanuric acid in which no deposits of cyanuric acid on the inner wall of the reactor occur and in which the necessity of removing such deposits mechanically does not therefore exist.

A further object is the production of cyanuric acid having a high degree of purity.

These objects are achieved by the method according to this invention by which the reaction mixture containin cyanuric acid and an auxiliary liq 'd is treated with a solution of a substance selected from the group consisting of otassium hydroxide, sodium hydroxide, potassium acetate, potassium carbonate, potassium formate, sodium acetate, sodium carbonate and sodium formate in a solvent selected from the group consisting of water and a mixture of Water with an alcohol, e.g. methanol, ethanol or isopropanol. By this treatment a cyanurate solution is formed. This is separated from the auxiliary liquid which again is recycled to the reactor. it is preferabl to use an aqueous alkali hydroxide soiu "on containing less than 25% by weight or" alkali hydroxide. This prevents the cyanurate solution from becoming so concen trated that upon cooling the pipes and vessels are clogged by deposits of alkali cyanurate.

In the practice of our invention cyanuric acid is made in conventional manner by heating urea in diphenyl oxide, dip-henyl or a mixture or" diphenyl oxide and diphenyl to a temperature at which cyanuric acid is formed from urea. The relative proportions of the two components when a mixture or" diphenyl oxide and diphenyl is used, may vary Within wide limits from 1 to 99% (by weight) for either component. The temperature is above 2 C., pre'fera ly between 220 and 279 C. As disclosed in our US. Patent No. 3,951,708, issued August 28, 1962, and in our German Patent No. l,064,522, granted September 3, 1959, the temperature should be not greater than 309 C. because, above this temperature, byroducts are formed which contaminate the cyanuric aci too greatly.

It is not necessary to convert the cyanuric acid completely into the alkali salt thereof, i.e., all hydroxy groups 5 the cyanuric acid need not be reacted with the abovementioned alkaline media. However it is advantageous to use the alkali hydroxide or the alkali salt of a weal: acid, such as potassium or sodium acetate, carbonate or formats, in such an amount that the cyanuric acid is substantially converted to the trialkali cyanurate because the trialkali cyanurates have a good solubility in water and mixtures of water and alcohol.

When the solvent used is a mixture of water and alcohol, the amount of alcohol contained therein should only be such as ensure that the alkali compounds are dissolved well. The cyanurate formed by the reaction with cyanuric acid must also dissolve well in the mixture. The aqueous cyanurate solution can be further processed Without special purification, for example for the production of trior di-chlorcyanuric acid. However free cyanuric acid may also be obtained by aci ifying the solution, for example with mineral acids.

Compared with the conventional methods, the production of cyanuric acid by the method according to this invention is greatly simplified. Moreover the purity of the cyanuric acid obtained is improved. The content of the pure substance in the end product is 98% by weight or more.

The process may be carried out continuously or batchwise. The formation of the cyanurate may be effected at normal or increased pressure and temperature.

The following examples wil further illustrate this invention but the invention is not restricted to these exampics. The parts and percentages specified in the examples are parts by Weight.

Example 1 1,000 parts of diphenyl oxide is placed in a 3-rn. stainless steel reactor and then 789 parts of urea is added evenly through a metering device during the course of 2 hours. The temperature of the reaction mixture is kept at 245 to 258 C. During reaction 190 parts of ammonia and 35 parts of ammonium salts, mainly ammonium carbonate escape. After the reaction has ended, the reactor and its contents are cooled to 100 C. and the reactor is con nected by a pipe to an San-. vessel containing 5,310 parts of an 8.8% aqueous caustic soda solution. The reactor and the vessel are interconnected by a circulating pump so that a closed circulatory system is established. The caustic soda solution is recirculated for about an hour, the whole of the cyanuric acid passing into solution. The aqueous cyanurate solution and the diphenyl oxide are then allowed to flow into a reservoir and the organic phase is separated from the aqueous phase. The diphenyl oxide is returned to the reactor While the aqueous cyanurate solution is pumped through a carbon filter and then adjusted to 3 to 4 in a stirrin vessel by the addition of hydrochloric acid. The cyauuric acid which deposits out is separated from the liquid, washed with water and dried. There are obtained 500 parts of colorless cyanuric acid Wnh a purity of about 99%. The yield is of the theory.

Example 2 Example 3 1300 parts of di henyl is placed in a stainless steel reactor and to this liquid 78% parts of urea is added evenly over a period of two and one half hours, the pressure being ntained at 370 Hg during the urea addition. The temperature or" the reaction mixture is 220 to 225 C. Dur ng reaction there are formed 182 parts of ammonia and 41 parts of ammonium salts, mainly ammonium carbonate, which escape from the reaction mixture in gaseous form. When all of the urea has been added, the reaction mixture is stirred'for another 20 minutes and then cooled to about 60 C. The cyanuric acid formed during reaction is available in the reactor as a solid. Then 5,320 parts of a 8.7% sodium hydroxide solution are run into the reaction mixture from a reservoir and the mixture of diphenyl and aqueous sodium hydroxide solution is recir- Example 4 1,000 parts of a mixture of 80% of diphenyl and 20% 0t diphenyl oxide is placed in a stainless steel reactor and to this mixture 760 parts of urea is added evenly over a period of one and three quarters of an hour, the temperature of the reaction mixture being maintained at 265 to 27G C. during reaction. The reaction mixture is under an excess pressure of 0.5 atmosphere. vhen all of the urea has been added, the reaction mixture is stirred for 10 minutes. The reaction mixture is added with 5,320 parts of an 8.7% aqueous sodium hydroxide solution and then recirculated as in Example 1. After stripping the organic liquid, precipitating the yanuric acid from the aqueous solution and drying it, there is obtained 503 parts or" a white powder of which 99.6% is cyanuric acid.

Analysis.Found: C=27.6%; H=2.6%; 0:37.070; N=32.9%. Calculated: C=27.9%; H=2.4%; O=37.2%; N:32.6%.

This application is a continuation-in-part of our copending application Serial No. 842,198 of September 25, 1959, now abandoned.

What We claim is:

1. In a process for the production of cyanuric acid by heating urea to a temperature above 200 C. and up to 300 C. in the presence of a substance selected from the group consisting of dipheuyl oxide, diphenyl and a mixture of diphenyl oxide and diphenyl, the improvement which comprises treating the reaction mixture containing cyanuric acid with a solution of an alkali compound seselected from the group consisting of potassium hydroxide, sodium hydroxide, potassium acetate, potassium carbonate, potassium formate, sodium acetate. sodium carbonate, and sodium formate in a solvent selected from the group consisting of water and a m'mture of water and alcohol and separating from the auxiliary liquid an aqueous solution of the alkali cyanurate formed by the reaction of the cyanuric acid and the alkali compound.

2. A process as claimed in claim 1, wherein an aqueous solution is used which contains less than 25% by weight of alkali hydroxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,872,447 Oehlschlaeger Feb. 3, 1959 3,051,708 Merkel et al. Aug. 28, 1962 FOREIGN PATENTS 551,749 Canada Jan. 14, 1958 

1. IN A PROCESS FOR THE PRODUCTION OF CYANURIC ACID BY HEATING UREA TO A TEMPERATURE ABOVE 200*C. AND UP TO 300*C. IN THE PRESENCE OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF DIPHENYL OXIDE, DIPHENYL AND A MIXTURE OF DIPHENYL OXIDE AND DIPHENYL, THE IMPROVEMENT WHICH COMPRISES TREATING THE REACTION MIXTURE CONTAINING CYANURIC ACID WITH A SOLUTION OF AN ALKALI COMPOUND SESELECTED FROM THE GROUP CONSISTING OF POTASSIUM HYDROXIDE, SODIUM HYDROXIDE, POTASSIUM ACETATE, POTASSIUM CARBONATE, POTASSIUM FORMATE, SODIUM ACETATE, SODIUM CARBONATE, AND SODIUM DORMATE IN A SOLVENT SELECTED FROM THE GROUP CONSISTING OF WATER AND A MIXTURE OF WATER AND ALCOHOL AND SEPARATING FROM THE AUXILIARY LIQUID AN AQUEOUS SOLUTION OF THE ALKALI CYANURATE FORMED BY THE REACTION OF THE CYANURIC ACID AND THE ALKALI COMPOUND. 